This invention relates to separators for use between electrodes within high temperature electrochemical cells employing molten electrolytic salts as electrolyte. Of particular interest is enhanced wettability of these interelectrode separators by molten salts such as the alkali metal halides or the alkaline earth metal halides.
A substantial amount of work has been done in the development of high temperature electrochemical cells. Cells of this type are contemplated for electric vehicle propulsion and for off-peak power storage. The cells contemplated can include such as the chalcogens or transition metal chalcogenides in the positive electrode and an alkali metal such as lithium, sodium or alloys of these metals with less reactive elements within the negative electrode. A typical high temperature electrochemical cell may include FeS in the positive electrode, LiAl in the negative electrode and an electrolyte of LiCl-KCl.
Compact cell design is desirable to provide high capacity and power per unit cell volume or weight. In compact cells an electrically insulative but porous separator is placed between the positive and negative electrodes to prevent shorting. The interelectrode separator should be wetted with electrolyte to ensure ionic conduction between the electrodes.
Boron nitride has been found to be an excellent choice for the interelectrode separator from the standpoint of compatibility with the cell environment at the high temperatures and for its good electrical insulative qualities. Unfortunately, boron nitride is extremely difficult to wet with the various molten salt electrolytes that are desired for use within these high temperature cells.
Various prior methods have been suggested for enhancing the wettability of boron nitride thereby making it more acceptable for use in molten salt electrochemical cells. Such methods have involved adding agents that enhance wettability in solid mixture with the separator material or by the application of a liquid or flux onto the boron nitride surfaces. In these methods various problems have been encountered control of loading, premature loss of the agent and the formation of conductive bridges within the separator.
Representative literature illustrating the background of the present development are as follows:
U.S. Pat. No. 4,189,827, Treatment of Electrochemical Cell Components with Lithium Tetrachloraluminate (LiAlCl.sub.4) to Promote Electrolyte Wetting by Eberhard et al. This patent shows the use of LiAlCl.sub.4 treatment prior to contact of the interelectrode separator material with the molten electrolytic salt.
U.S. Pat. No. 3,898,096 "Lithium-Molten Salt Cell with Transition Metal Chalogenide Positive Electrode" to Heredy and McCoy illustrates various active materials contemplated for use in electrochemical cells employing the various molten salt electrolytes.
U.S. Pat. No. 4,110,517 "Electrochemical Cell Design" to Arntzen illustrates a compact electrochemical cell design that can contain a boron nitride felt as an interelectrode separator.